FAT FILESYSTEM

FAT

The traditional DOS filesystem types are FAT12 and FAT16. Here FAT stands for File Allocation Table: the disk is divided into clusters, the unit used by the file allocation, and the FAT describes which clusters are used by which files.

Layout
First the boot sector (at relative address 0), and possibly other stuff. Together these are the Reserved Sectors. Usually the boot sector is the only reserved sector.
Then the FATs (following the reserved sectors; the number of reserved sectors is given in the boot sector, bytes 14-15; the length of a sector is found in the boot sector, bytes 11-12).
Then the Root Directory (following the FATs; the number of FATs is given in the boot sector, byte 16; each FAT has a number of sectors given in the boot sector, bytes 22-23).
Finally the Data Area (following the root directory; the number of root directory entries is given in the boot sector, bytes 17-18, and each directory entry takes 32 bytes; space is rounded up to entire sectors).

Boot sector

The first sector (512 bytes) of a FAT filesystem is the boot sector. In Unix-like terminology this would be called the superblock. It contains some general information.
First an explicit example (of the boot sector of a DRDOS boot floppy).

0000000 eb 3f 90 49 42 4d 20 20 33 2e 33 00 02 01 01 00
0000020 02 e0 00 40 0b f0 09 00 12 00 02 00 00 00 00 00
0000040 00 00 00 00 00 00 00 00 00 00 70 00 ff ff 49 42
0000060 4d 42 49 4f 20 20 43 4f 4d 00 50 00 00 08 00 18
...

(See here for the complete sector. And also a MSDOS example)The 2-byte numbers are stored little endian (low order byte first).
Here the FAT12 version, that is also the common part of the FAT12, FAT16 and FAT32 boot sectors. See further below.

Bytes   Content
0-2     Jump to bootstrap (E.g. eb 3c 90; on i86: JMP 003E NOP.
        One finds either eb xx 90, or e9 xx xx.
        The position of the bootstrap varies.)
3-10    OEM name/version (E.g. "IBM  3.3", "IBM 20.0", "MSDOS5.0", "MSWIN4.0".
        Various format utilities leave their own name, like "CH-FOR18".
        Sometimes just garbage. Microsoft recommends "MSWIN4.1".)
        /* BIOS Parameter Block starts here */
11-12   Number of bytes per sector (512)
        Must be one of 512, 1024, 2048, 4096.
13      Number of sectors per cluster (1)
        Must be one of 1, 2, 4, 8, 16, 32, 64, 128.
        A cluster should have at most 32768 bytes. In rare cases 65536 is OK.
14-15   Number of reserved sectors (1)
        FAT12 and FAT16 use 1. FAT32 uses 32.
16      Number of FAT copies (2)
17-18   Number of root directory entries (224)
        0 for FAT32. 512 is recommended for FAT16.
19-20   Total number of sectors in the filesystem (2880)
        (in case the partition is not FAT32 and smaller than 32 MB)
21      Media descriptor type (f0: 1.4 MB floppy, f8: hard disk; see below)
22-23   Number of sectors per FAT (9)
        0 for FAT32.
24-25   Number of sectors per track (12)
26-27   Number of heads (2, for a double-sided diskette)
28-29   Number of hidden sectors (0)
        Hidden sectors are sectors preceding the partition.
        /* BIOS Parameter Block ends here */
30-509  Bootstrap
510-511 Signature 55 aa

The signature is found at offset 510-511. This will be the end of the sector only in case the sector size is 512.